A buffer is generally a solution containing both a weak acid and its conjugate weak base whose pH changes only slightly on addition of acid or alkali. The weak acid becomes a buffer when alkali is added and the weak base becomes a buffer on addition of acid. This action can be explained by the reaction: EQU A+HOH.fwdarw.B+H.sub.3 O.sub.n ( 1)
in which the base B is formed by the loss of proton from the corresponding acid A. The acid may be a cation such as NH.sub.4.sup.+, a neutral molecule such as CH.sub.3 COOH, or an anion such as H.sub.2 PO.sub.4.sup.-. When alkali is added, hydrogen ions are removed to form water, but as long as the added alkali is not in excess of the buffer acid, many of the hydrogen ions are replaced by further ionization of the acid to maintain the equilibrium. When acid is added, this reaction is reversed as hydrogen ions combine with the base to form acid.
Buffers of this type are used in many applications. One specific application is the use of a bicarbonate as a buffering composition in a fiber optic physiological probe, such as the one described in U.S. Pat. No. 4,925,268 issued May 15, 1990 and U.S. Pat. No. 5,000,901 issued Mar. 19, 1991. The probes described in these two patents include a carbon dioxide sensor that relies upon the optical absorbence of an indicator material, such as phenol red (PR) in its basic form, to provide a measure of carbon dioxide in the analyte. Generally, the degree that phenol red absorbs visible light is affected by the pH of the environment around the phenol red. As described below, in the carbon dioxide sensors of the noted patents, the pH of the environment around the phenol red is made dependent upon the concentration of carbon dioxide around the sensor, by relying upon a buffering composition. The buffering solution responds to changes in the concentration of carbon dioxide by changing the pH of the environment in which it is contained.
In the sensors described in the above-noted patents, the phenol red is codissolved in a matrix of methyl methacrylate (MMA), methacrylamidopropyltrimethylammonium chloride (MAPTAC), and polyethylene glycol (PEG) having a molecular weight of about 600 K. The matrix also incorporates a base that establishes an equilibrium within the matrix, such that when the concentration of carbon dioxide in the matrix changes, the pH within the matrix changes. The change in pH affects the absorbence of phenol red. One type of buffer described in the above-noted patents comprises a bicarbonate ion. The bicarbonate ion is incorporated into the matrix in the form of an inorganic salt, such as a sodium salt. Bicarbonate sets up the following equilibrium that is sensitive to the carbon dioxide concentration: EQU CO.sub.2 +H.sub.2 O.revreaction.H.sub.2 CO.sub.3 .revreaction.HCO.sub.3.sup.- +H.sup.+ ( 2)
In the probe described in the above-noted patents, a matrix of MMA/MAPTAC/PR/PEG is applied to the end of an optical fiber by using a solvent based film coating technique. The bicarbonate can then be introduced into the matrix by diffusion, so that a coating of the MMA/MAPTAC/PR/PEG matrix impregnated with sodium bicarbonate covers the end of the optical fiber. The sensor matrix can be further encapsulated by a layer of a plastic which forms a gas permeable, but ion impermeable membrane around the matrix. Visible light is directed onto the sensor matrix through the optical fiber. The absorbence of the impinging light by the phenol red can be measured by detecting the light reflected by the sensor matrix.
When the probe is designed to monitor carbon dioxide concentrations in blood, the accuracy of the measurement is vitally important. In the past, the possibility of bicarbonate ions leaking out of the sensor matrix increased the likelihood that signal drift would occur. Leaking of the bicarbonate ions from the sensor matrix could result if small voids occurred in the plastic membrane encapsulating the sensor matrix.
U.S. Pat. Nos. 5,000,901 and 4,925,268 teach that as an alternative to sodium bicarbonate, other bases having a pKa in the targeted physiological range can be used. Suitable monomeric bases that are listed include 2-vinylpyridine, 4-vinylpyridine, histamine, 1-vinylimidazole and 4-vinylimidazole. These patents note that the vinylic monomers can be homopolymerized or copolymerized to provide a polymeric base of sufficiently high molecular weight to nullify loss by permeation when physically entrained in the sensor matrix. In order to immobilize the base by covalent linkage to the sensor matrix, the monomers are described as being copolymerizable with MMA or otherwise bondable to a resin emulsion. The disclosed monomeric bases have a pKa that is near the low end of the physiological range and accordingly, compared to bicarbonate, they are not as effective at providing a sensor that has the desired sensitivity and response time. Accordingly, while the polymeric bases described above may avoid the potential problem of leakage of the base from the sensor matrix and the consequent signal drift, they are less desirable from an accuracy and speed standpoint.
The buffering composition formed in accordance with the present invention overcomes the problems of using free bicarbonate ions as a buffer in a sensor matrix such as that described above to monitor carbon dioxide concentrations. The composition of the present invention avoids the potential leakage and signal drift problems associated with the use of free bicarbonate without a sacrifice in performance in demanding applications such as physiological probes.